This invention relates to gas, liquid and vapor sensors and more particularly, to such sensors which use a swellable member as the means for sensing the gas, liquid or vapor.
There is a desire to reduce emissions of organic chemicals in the process control and other industries. Refineries and intermediate users such as pumping stations and storage facilities emit large amounts of undesired organic chemicals. Progressively more stringent government regulations have addressed this problem, most notably the U.S. federal regulations 37 CFR 191-192 and regulations from other states as well. Such rules regulate emissions from "release points" and mandate monitoring for fittings, joints, packing glands, flanges, valves, pump seals, compressor seals, pressure relief seals, open-ended lines and sampling connections. Valves, because of their number and relatively high emission rates, are the major emission source. The U.S. Environmental Protection Agency (EPA) Office of Air Quality sets limits on emissions from various "release points", as published in "Protocols for Generating Unit-Specific Emission Estimate for Equipment Leaks of VOC and VHAP", EPA publication 450/3-88-010.
Organic chemical sensors formed of polymers, such as a room-temperature-vulcanized (RTV) silicones impregnated with electrically conductive particles to form a variable resistor are known. Exposure to organic chemicals swells or increases the volume of the polymer which moves the particles farther apart and increases electrical resistivity. Such sensors, however, can have poor stability and repeatability because the organic chemicals damage the particles. Also, where an "intrinsically safe" design is needed for explosive vapors, such sensors are unsuitable because they carry an electric current and may spark under fault conditions. Further, outputs from such sensors are typically low level and require a Wheatstone bridge circuit for sensing. The high impedance bridge signals are noisy and transmission over long distance requires extensive shielding and amplification.
There is thus a need for an inexpensive organic chemical sensor which has a more stable and repeatable output which can be used in the presence of explosive vapors and which are more suitable for transmission in a noisy process control environment.